The present invention relates to a curable white ink composition employed in an ink-jet recording system.
By employing the ink-jet recording system, it is possible to easily and conveniently produce images at low cost. Further, along with recent improvement in image quality, it has received attention as a technique which enables high image quality recording which is sufficiently applicable to various printing fields.
Due to its printing mechanism, generally employed as ink-jet ink compositions are low viscosity ink compositions comprising aqueous solvents or non-aqueous solvents as a main component. Due to that, recording media are preferably ink-adsorptive so that ink droplets which form image dots result in no two-dimensional spread. Specifically, in order to achieve high image quality, special paper is needed.
Contrary to this, proposed as ink compositions capable of being printed onto non-ink absorptive recording media such as film and metal through adhesion are, for example, an ink composition comprised of a component which undergoes polymerization by UV irradiation, described in Japanese Patent Application Open to Public Inspection No. 3-216379, and a UV curable type ink composition comprised of colorants, UV curable agents, and photopolymerization initiators, described in U.S. Pat. No. 5,623,001. Conventionally, however, multifunctional acrylate based compounds employed in UV curable type inks tend to be subjected to curing inhibition in the presence of oxygen during curing. Further, safety concerns exist in that most of them are toxic to skin and result in a rash, and in fact there are few safe compounds. Still further, problems occur in which recording media, when employed as a thin layer substrate for soft packaging or an adhesive label, suffer from shrinkage due to high contraction ratios during curing. As a result, their application has been limited.
Still further, most of the common ink-jet inks are highly transparent inks which are employed for printing onto white recording media. For example, when printing is carried out employing recording media such as a transparent substrate employed in soft packaging or a low lightness substrate, occasionally it is difficult to achieve the desired contrast, as well as bright color formation, and markings with desired visibility.
In the case of poor visibility, a method is known in which desired visibility is achieved employing a high covering white ink. For example, Japanese Patent Publication No. 2-45663 describes a white ink composition which is comprised of inorganic white pigments, organic solvents and binding resins at a viscosity of 1–5 mPa·S at 5–40° C., and Japanese Patent Application Open to Public Inspection No. 2000-336295 proposes a light curable type ink-jet recording ink composition which is comprised of polymerizable compounds, photopolymerization initiators, and aqueous solvents.
However, since the viscosity of these ink compositions is relatively low at room temperature, ejected ink dots spread onto non-ink absorptive media. In addition, since the aforesaid ink compositions comprise solvents, thermal drying is needed to remove residues. As a result, the aforesaid ink compositions are not suitable for printing onto recording media such as thermally shrinkable substrates.